Hypolithons are protected from harsh ultraviolet radiation and wind scouring by their rock, which can also trap moisture. The rocks are generally translucent to allow for the penetration of light. Writing in Nature, ecologist Charles S. Cockell of the British Antarctic Survey suggests that all hypoliths reported to date have been found under quartz, which is one of the most common translucent rocks (Nature 431, 414 (23 September 2004); doi:10.1038/431414a).
However, Cockell reported that on Cornwallis Island and Devon Island, 94-95% of a random sample of 850 opaquedolomitic rocks were colonized by hypoliths, and found that the communities were dominated by cyanobacteria. The rocks chosen were visually indistinguishable from those nearby, and were about 10cm across; the hypolithon was visible as a greenish coloured band. Cockell proposed that rock sorting by periglacial action, including that during freeze–thaw cycles, improves light penetration around the edges of rocks (see granular material and Brazil nut effect).
Cockell went on to estimate the productivity of the Arctic communities by monitoring the uptake of sodium bicarbonate labelled with Carbon-14 and found that (for Devon Island) productivity of the hypolithon was comparable to that of plants, lichens, and bryophytes combined ( and respectively) and concluded that the polar hypolithon may double previous estimates of the productivity of the rocky polar desert.
Sample 11-18a was a candidate hypolith from beneath a rock embedded in the streambed above Lith Canyon where we disembarked from the ATVs.
The moist patch of soil beneath the rock contained a much lighter area that suggested mat material, and it is from this that the bulk of the sample was retrieved.
It was quite different from 11-18a, however, in that as opposed to a large patch of potentially sand material, it resembled a dollop of grey paste about two cm in size and elevated about 2 mm from the cavity of the rock that had previously been covering it.
Writing in Nature, ecologist Charles S. Cockell of the British Antarctic Survey and Dale Stokes describe how hypoliths reported to date (until 2004) had been found under quartz, which is one of the most common translucentrocks (Nature 431, 414 (23 September 2004); doi:10.1038/431414a).
Share your thoughts, questions and commentary here
Want to know more? Search encyclopedia, statistics and forums:
Press Releases |
The Wikipedia article included on this page is licensed under the
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms, 1022, m